Heat carrier medium for magnetocaloric materials

ABSTRACT

What is described is the use of alcohols, alcoholamines, diols, polyols or mixtures thereof in heat carrier media or as heat carrier media which are in contact with magnetocaloric materials.

The invention relates to the use of corrosion-stabilizing additives inheat carrier media or as heat carrier media which are in contact withmagnetocaloric materials, to corresponding heat carrier media and tomagnetic coolers, magnetic heat pumps or magnetic generators comprisingthem.

Magnetocaloric materials, also referred to as thermomagnetic materials,can be used in magnetic cooling, in heat pumps or air conditioningsystems, and generators.

Such materials are known in principle and are described, for example, inWO 2004/068512. Magnetic cooling techniques are based on themagnetocaloric effect (MCE) and may constitute an alternative to theknown vapor circulation cooling methods. In a material which exhibits amagnetocaloric effect, the alignment of randomly aligned magneticmoments by an external magnetic field leads to heating of the material.This heat can be removed from the MCE material to the surroundingatmosphere by a heat transfer. When the magnetic field is then switchedoff or removed, the magnetic moments revert back to a randomarrangement, which leads to cooling of the material below ambienttemperature. This effect can be exploited for cooling purposes; see alsoNature, Vol. 415, Jan. 10, 2002, pages 150 to 152. Typically, a heattransfer medium such as water is used for heat removal from themagnetocaloric material.

Customary materials are prepared by solid phase reaction of the startingelements or starting alloys for the material in a ball mill, subsequentpressing, sintering and heat treatment under inert gas atmosphere andsubsequent slow cooling to room temperature. Processing by means of meltspinning is also possible. This makes possible a more homogeneouselement distribution, which leads to an improved magnetocaloric effect.

A problem in the case of use of the heat exchange medium or heattransfer media is the corrosion tendency of the magnetocaloricmaterials. Attempts are being made in different ways to prevent thiscorrosion. Corrosion is also referred to as fouling or leaching.

Specifically the washout of toxic metals such as arsenic or manganese isproblematic. In general, the application properties of themagnetocaloric materials suffer as a result of corrosion, fouling orleaching.

US 2007/0220901 describes providing an oxidation-resistant film on thesurface of the magnetic material particles. This film is especiallyaluminum oxide or aluminum nitride.

JP-A-2006-124783 describes the coating of the magnetic particles with achemical film based on phosphoric acid for corrosion protection.

JP-A-2005-226125 describes metal plating for coating of magneticparticles, the intention being that ion plating with acorrosion-resistant metal should improve the corrosion protection.

These methods envisage complex further treatment of the magnetocaloricmaterials.

It is an object of the present invention to provide anticorrosives formagnetocaloric materials which avoid coating of the magnetocaloricmaterials and can also be integrated in a simple manner into alreadyexisting magnetic coolers, magnetic heat pumps or magnetic generators.

The object is achieved in accordance with the invention by use ofpreferably water-miscible alcohols, alcoholamines, diols, polyols ormixtures thereof in preferably aqueous heat carrier media or as heatcarrier media which are in contact with magnetocaloric materials. Thealcohols, diols, polyols or mixtures thereof serve as an anticorrosivefor prevention of corrosion of the magnetocaloric materials.

It has been found in accordance with the invention that the corrosion ofthe magnetocaloric materials can be greatly reduced when alcohols,alcoholamines, diols or polyols are used as heat carrier media or inpreferably aqueous heat carrier media.

In addition, the object is achieved in accordance with the invention byuse of aqueous liquids which have a pH of at least 8 at 25° C. as a heatcarrier medium which is in contact with magnetocaloric materials.

It has been found in accordance with the invention that, at basic pHvalues, the corrosion of the magnetocaloric materials can be greatlyreduced. Particularly good effects are achieved when aqueous heatcarrier media which comprise alcohols, diols, polyols or mixturesthereof and simultaneously have a pH of at least 8 are used.

The pH in the aqueous liquids is preferably at least 10, more preferablyat least 12, especially 12 to 14, at 25° C.

The water miscibility of the alcohols, alcoholamines, diols and polyolsmust, in the case of additional use of water as a heat carrier medium,be sufficiently great as to give rise to a homogeneous mixture orsolution at the desired mixing ratio with water. If only small amountsof alcohol should be envisaged, it is possible to switch to lower watermiscibility alcohols. At high alcohol contents, a good water miscibilityshould be ensured.

The alcohols used are preferably C₁₋₆-alkanols, more preferablymethanol, ethanol, n-propanol, 2-propanol or mixtures thereof.

Preferred alcoholamines are C₁₋₆-alkanolamines, especially ethanolamine.

The diols used are preferably C₂₋₆-alkanediols, especially ethyleneglycol, propylene glycol, butanediol or mixtures thereof.

Particularly preferred polyols have an aliphatic hydrocarbon radicalhaving 3 to 6 hydroxyl groups.

Particular preference is given to using ethanol and glycol.

The alcohols, alcoholamines, diols, polyols or mixtures thereof may beused as the sole heat carrier media or be introduced in any suitableamounts into aqueous heat carrier media. The content in the aqueous heatcarrier media of alcohols, alcoholamines, diols, or polyols or mixturesthereof is preferably 10 to 90% by weight, more preferably 10 to 70% byweight, especially 20 to 50% by weight.

According to the invention, particular preference is given to usingaqueous heat carrier media with a content of C₁₋₃-alkanoles orC₂₋₄-alkanediols in the range from 10 to 90% by weight, or the purealkanols or diols.

The magnetocaloric materials are preferably part of a magnetic cooler,of a magnetic heat pump or of a magnetic generator. For a description ofcustomary magnetic coolers, magnetic heat pumps or magnetic generators,reference may be made to the literature mentioned at the outset. Inaddition, WO 2006/074790 can be cited for a description of a magneticregenerator.

According to the invention, it is possible to use any desiredmagnetocaloric materials.

Typical magnetocaloric materials are multimetal materials which oftencomprise at least three metallic elements and additionally optionallynonmetallic elements. The expression “metal-based materials” or“magnetocaloric materials” indicates that the predominant proportion ofthese materials is formed from metals or metallic elements.

Typically, the proportion in the overall material is at least 50% byweight, preferably at least 75% by weight, especially at least 80% byweight. Suitable metal-based materials are explained in detail below.

The magnetocaloric material is more preferably selected from

(1) compounds of the general formula (I)

(A_(y)B_(y−1))_(2+δ)C_(w)D_(x)E_(Z)  (I)

-   -   where    -   A is Mn or Co,    -   B is Fe, Cr or Ni,    -   C, D and E at least two of C, D and E are different, have a        non-vanishing concentration and are selected from P, B, Se, Ge,        Ga, Si, Sn, N, As and Sb, where at least one of C, D and E is        Ge, As or Si,    -   δ is a number in the range from −0.1 to 0.1,    -   w, x, y, z are numbers in the range from 0 to 1, where w+x+z=1;

(2) La— and Fe-based compounds of the general formulae (II) and/or (III)and/or (IV)

Le(Fe_(x)Al_(1−x))₁₃H_(y) or La(Fe_(x)Si_(1−x))₁₃H_(y)  (II)

where

-   -   x is a number from 0.7 to 0.95,    -   y is a number from 0 to 3, preferably from 0 to 2;

La(Fe_(x)Al_(y)Co_(z))₁₃ or La(Fe_(x)Si_(y)Co_(z))₁₃  (III)

-   -   where    -   x is a number from 0.7 to 0.95,    -   y is a number from 0.05 to 1−x,    -   z is a number from 0.005 to 0.5;

LaMn_(x)Fe_(2−x)Ge  (IV)

-   -   where    -   x is a number from 1.7 to 1.95 and

(3) Heusler alloys of the MnTP type where T is a transition metal and Pis a p-doping metal having an electron count per atom e/a in the rangefrom 7 to 8.5.

Materials particularly suitable in accordance with the invention aredescribed, for example, in WO 2004/068512, Rare Metals, Vol. 25, 2006,pages 544 to 549, J. Appl. Phys. 99,08Q107 (2006), Nature, Vol. 415,Jan. 10, 2002, pages 150 to 152 and Physica B 327 (2003), pages 431 to437.

In the aforementioned compounds of the general formula (I), C, D and Eare preferably identical or different and are selected from at least oneof P, Ge, Si, Sn, As and Ga.

The metal-based material of the general formula (I) is preferablyselected from at least quaternary compounds which, as well as Mn, Fe, Pand if appropriate Sb, additionally comprise Ge or Si or As or Ge and Sior Ge and As or Si and As, or Ge, Si and As.

Preferably at least 90% by weight, more preferably at least 95% byweight, of component A is Mn. More preferably at least 90% by weight,more preferably at least 95% by weight, of B is Fe. Preferably at least90% by weight, more preferably at least 95% by weight, of C is P.Preferably at least 90% by weight, more preferably at least 95% byweight, of D is Ge. Preferably at least 90% by weight, more preferablyat least 95% by weight, of E is Si.

The material preferably has the general formula MnFe(P_(w)Ge_(x)Si_(z)).

x is preferably a number in the range from 0.3 to 0.7, w is less than orequal to 1−x and z corresponds to 1−x−w.

The material preferably has the crystalline hexagonal Fe₂P structure.Examples of suitable structures are MnFeP_(0.45 to 0.7),Ge_(0.55 to 0.30) and MnFeP_(0.5 to 0.70), (Si/Ge)_(0.5 to 0.30).

Suitable compounds are additionally M_(n1+x)Fe_(1−x)P_(1−y)Ge_(y) with xin the range from −0.3 to 0.5, y in the range from 0.1 to 0.6. As mayalso be present in place of Ge. Likewise suitable are compounds of thegeneral formula Mn_(1+x)Fe_(1−x)P_(1−y)Ge_(y−1),Sb_(z) with x in therange from −0.3 to 0.5, y in the range from 0.1 to 0.6 and z less than yand less than 0.2. Also suitable are compounds of the formulaMn₁₊xFe_(1−x)P_(1−y)Ge_(y−z)Si_(z)with x in the range from 0.3 to 0.5, yin the range from 0.1 to 0.66, z less than or equal to y and less than0.6. Also advantageous are compounds of the Mn_(x)Fe_(2−x)P_(y)As_(1−y)type where x=0.7 to 1.3 and y=0.3 to 0.7.

Preferred La— and Fe-based compounds of the general formulae (II) and/or(III) and/or (IV) are La(Fe_(0.90)Si_(0.10))₁₃,La(Fe_(0.89)Si_(0.11))₁₃, La(Fe_(0.880)Si_(0.120) ₁₃,La(Fe_(0.877)Si_(0.123))₁₃, LaFe_(11.8)Si_(1.2),La(Fe_(0.88)Si_(0.12))₁₃H_(0.5), La(Fe_(0.88)Si_(0.12))₁₃H_(1.0),LaFe_(11.7)Si_(1.3)H_(1.1), LaFe_(11.57)Si_(1.43)H_(1.3, LaFe(Fe)_(0.88)Si_(0.12))H_(1.5), LaFe_(11.2)Co_(0.7)Si_(1.1),LaFe_(11.5)Al_(1.5)C_(0.1), LaFe_(11.5)Al_(1.5)C_(0.2),LaFe_(11.5)Al_(1.5)C_(0.4), LaFe_(11.5)Al_(1.5)Co_(0.5),La(Fe_(0.94)Co_(0.06))_(11.83)Al_(1.17),La(Fe_(0.92)Co_(0.08))_(11.83)Al_(1.17).

Suitable manganese-comprising compounds are MnFeGe, MnFeGe,MnFe_(0.9)Co_(0.1)Ge, MnFe_(0.8)Co_(0.2)Ge, MnFe_(0.7)Co_(0.3)Ge,MnFe_(0.6)Co_(0.4)Ge, MnFe_(0.5)Co_(0.5)Ge, MnFe_(0.4)Co_(0.6)Ge,MnFe_(0.3)Co_(0.7)Ge, MnFe_(0.2)Co_(0.8)Ge, MnFe_(0.15)Co_(0.85)Ge,MnFe_(0.1)Co_(0.9)Ge, MnCoGe, Mn₅Ge_(2.5)Si_(0.5), Mn₅Ge₂Si,Mn₅Ge_(1.5)Si¹⁻⁵, Mn₅GeSi₂, Mn₅Ge₃, Mn₅Ge_(2.95) l Sb_(0.1),Mn₅Ge_(2.8)Sb_(0.2), Mn₅Ge_(2.7)Sb_(0.3), LaMn_(1.9)Fe_(0.1)Ge,LaMn_(1.85)Fe_(0.15)Ge, LaMn_(1.8)Fe_(0.2)Ge, (Fe_(0.9)Mn_(0.1))₃C,(Fe_(0.8)Mn_(0.2))₃C, (Fe_(0.7)Mn_(0.3))₃C, Mn₃GaC, MnAs, (Mn, Fe)As,Mn_(1+δ)As_(0.8)Sb_(0.2), MnAs_(0.75)Sb_(0.25),Mn_(1.1)As_(0.75)Sb_(0.25), Mn_(1.5)As_(0.75)Sb_(0.25).

Heusler alloys suitable in accordance with the invention are, forexample, Fe₂MnSi_(0.5)Ge_(0.5), Ni_(52.9)Mn_(22.4)Ga_(24.7),Ni_(50.9)Mn_(24.7)Ga_(24.4), Ni_(55.2)Mn_(18.6)Ga_(26.2),Ni_(51.6)Mn_(24.7)Ga_(23.8), Ni_(52.7)Mn_(23.9)Ga_(23.4), CoMnSb,CONb_(0.2)Mn_(0.8)Sb, CoNb_(0.4)Mn_(0.6)Sb, CoNb_(0.6)Mn_(0.4)Sb,Ni₅₀Mn₃₅Sn₁₅, Ni₅₀Mn₃₇Sn₁₃, MnFeP_(0.45)As_(0.55),MnFeP_(0.47)As_(0.53), Mn_(1.1)Fe_(0.9)P_(0.47)As_(0.53), MnFeP_(0.89−)_(x) Si_(x)Ge_(0.11), X=0.22, X=0.26, X=0.30, X=0.33.

The average crystal size is frequently in the range from 10 to 400 nm,more preferably 20 to 200 nm, especially 30 to 80 nm. The averagecrystal size can be determined by X-ray diffraction. When the crystalsize becomes too small, the maximum magnetocaloric effect is reduced.When the crystal size, in contrast, is too large, the hysteresis of thesystem rises.

The inventive metal-based materials are preferably used in magneticcooling as described above. A corresponding refrigerator has, inaddition to a magnet, preferably a permanent magnet, metal-basedmaterials as described above. The cooling of computer chips and solarpower generators is also possible. Further fields of application areheat pumps and air conditioning systems.

The metal-based materials prepared by the process according to theinvention may have any desired solid form. They may be present, forexample, in the form of flakes, ribbons, wires, powder, or else in theform of shaped bodies. Shaped bodies such as monoliths or honeycombs canbe produced, for example, by a hot extrusion process. It is possible,for example, for cell densities of 400 to 1600 CPI or more to bepresent. Thin sheets obtainable by rolling processes are also preferredin accordance with the invention. Advantageous nonporous shaped bodiesare those composed of shaped thin material, for example tubes, plates,meshes, grids or rods. Shaping by metal injection molding (MIM)processes is also possible in accordance with the invention.

The preparation of the metal-based materials for the magnetic cooling orheat pumps or generators may comprise the following steps:

a) reacting chemical elements and/or alloys in a stoichiometry whichcorresponds to the metal-based material in the solid and/or liquidphase,

b) if appropriate converting the reaction product from stage a) to asolid,

c) sintering and/or heat treating the solid from stage a) or b),

d) quenching the sintered and/or heat-treated solid from stage c) at acooling rate of at least 100 K/s.

The thermal hysteresis can be reduced significantly when the metal-basedmaterials are not cooled slowly to ambient temperature after thesintering and/or heat treatment, but rather are quenched at a highcooling rate. This cooling rate is at least 100 K/s. The cooling rate ispreferably from 100 to 10 000 K/s, more preferably from 200 to 1300 K/s.Especially preferred cooling rates are from 300 to 1000 K/s.

The quenching can be achieved by any suitable cooling processes, forexample by quenching the solid with water or aqueous liquids, forexample cooled water or ice/water mixtures. The solids can, for example,be allowed to fall into ice-cooled water. It is also possible to quenchthe solids with subcooled gases such as liquid nitrogen. Furtherprocesses for quenching are known to those skilled in the art. What isadvantageous here is controlled and rapid cooling.

Without being bound to a theory, the reduced hysteresis can beattributed to smaller particle sizes for the quenched compositions.

In alternative processes, the sintering and heat treatment are eachfollowed by slow cooling, which leads to the formation of largerparticle sizes and hence to an increase in thermal hysteresis.

In step (a) of the process, the elements and/or alloys which are presentin the later metal-based material are converted in a stoichiometry whichcorresponds to the metal-based material in the solid or liquid phase.

Preference is given to performing the reaction in stage a) by combinedheating of the elements and/or alloys in a closed vessel or in anextruder, or by solid phase reaction in a ball mill. Particularpreference is given to performing a solid phase reaction, which iseffected especially in a ball mill. Such a reaction is known inprinciple; cf. the documents cited by way of introduction. Typically,powders of the individual elements or powders of alloys of two or moreof the individual elements which are present in the later metal-basedmaterial are mixed in pulverulent form in suitable proportions byweight. If necessary, the mixture can additionally be ground in order toobtain a microcrystalline powder mixture. This powder mixture ispreferably heated in a ball mill, which leads to further comminution andalso good mixing, and to a solid phase reaction in the powder mixture.

Alternatively, the individual elements are mixed as a powder in theselected stoichiometry and then melted.

The combined heating in a closed vessel allows the fixing of volatileelements and control of the stoichiometry. Specifically in the case ofuse of phosphorus, this would evaporate easily in an open system.

The reaction is followed by sintering and/or heat treatment of thesolid, for which one or more intermediate steps can be provided. Forexample, the solid obtained in stage a) can be pressed before it issintered and/or heat treated. This allows the density of the material tobe increased, such that a high density of the magnetocaloric material ispresent in the later application. This is advantageous especiallybecause the volume within which the magnetic field exists can bereduced, which may be associated with considerable cost savings.Pressing is known per se and can be carried out with or without pressingaids. It is possible to use any suitable mold for pressing. By virtue ofthe pressing, it is already possible to obtain shaped bodies in thedesired three-dimensional structure. The pressing may be followed by thesintering and/or heat treatment of stage c), followed by the quenchingof stage d).

Alternatively, it is possible to send the solid obtained from the ballmill to a melt-spinning process. Melt-spinning processes are known perse and are described, for example, in Rare Metals, Vol. 25, October2006, pages 544 to 549, and also in WO 2004/068512.

In these processes, the composition obtained in stage a) is melted andsprayed onto a rotating cold metal roller. This spraying can be achievedby means of elevated pressure upstream of the spray nozzle or reducedpressure downstream of the spray nozzle. Typically, a rotating copperdrum or roller is used, which can additionally be cooled if appropriate.The copper drum preferably rotates at a surface speed of from 10 to 40m/s, especially from 20 to 30 m/s. On the copper drum, the liquidcomposition is cooled at a rate of preferably from 10² to 10⁷ K/s, morepreferably at a rate of at least 10⁴ K/s, especially with a rate of from0.5 to 2×10⁶ K/s.

The melt-spinning, like the reaction in stage a) too, can be performedunder reduced pressure or under an inert gas atmosphere.

The melt-spinning achieves a high processing rate, since the subsequentsintering and heat treatment can be shortened. Specifically on theindustrial scale, the production of the metal-based materials thusbecomes significantly more economically viable. Spray-drying also leadsto a high processing rate. Particular preference is given to performingmelt spinning.

Alternatively, in stage b), spray cooling can be carried out, in which amelt of the composition from stage a) is sprayed into a spray tower. Thespray tower may, for example, additionally be cooled. In spray towers,cooling rates in the range from 10³ to 10⁶ K/s, especially about 10⁴K/s, are frequently achieved.

The sintering and/or heat treatment of the solid is effected in stage c)preferably first at a temperature in the range from 800 to 1400° C. forsintering and then at a temperature in the range from 500 to 750° C. forheat treatment. These values apply especially to shaped bodies, whilelower sintering and heat treatment temperatures can be employed forpowders. For example, the sintering can then be effected at atemperature in the range from 500 to 800° C. For shaped bodies/solids,the sintering is more preferably effected at a temperature in the rangefrom 1000 to 1300° C., especially from 1100 to 1300° C. The heattreatment can then be effected, for example, at from 600 to 700° C.

The sintering is performed preferably for a period of from 1 to 50hours, more preferably from 2 to 20 hours, especially from 5 to 15hours. The heat treatment is performed preferably for a period in therange from 10 to 100 hours, more preferably from 10 to 60 hours,especially from 30 to 50 hours. The exact periods can be adjusted to thepractical requirements according to the materials.

In the case of use of the melt-spinning process, it is frequentlypossible to dispense with sintering, and the heat treatment can beshortened significantly, for example to periods of from 5 minutes to 5hours, preferably from 10 minutes to 1 hour. Compared to the otherwisecustomary values of 10 hours for sintering and 50 hours for heattreatment, this results in a major time advantage.

The sintering/heat treatment results in partial melting of the particleboundaries, such that the material is compacted further.

The melting and rapid cooling in stage b) thus allows the duration ofstage c) to be reduced considerably. This also allows continuousproduction of the metal-based materials.

Particular preference is given to the process sequence of

a) solid phase reaction of chemical elements and/or alloys in astoichiometry which corresponds to the metal-based material in a ballmill,

b) melt spinning the material obtained in stage a),

c) heat treating the solid from stage b) at a temperature in the rangefrom 430 to 1200° C., preferably from 800 to 1000° C., for a period offrom 10 seconds or 1 minute to 5 hours, preferably from 30 minutes to 2hours,

d) quenching the heat treated solid from stage c) at a cooling rate offrom 200 to 1300 K/s.

The invention also relates to a heat carrier medium for magneticcoolers, magnetic heat pumps, magnetic generators, comprising preferablywater-miscible alcohols, alcoholamines, diols, polyols or mixturesthereof.

The invention also relates to magnetic coolers, magnetic heat pumps ormagnetic generators, comprising at least one magnetocaloric material anda heat carrier medium, wherein the heat carrier medium compriseswater-miscible alcohols, alcoholamines, diols, polyols or mixturesthereof, or consists thereof, and/or has a pH of at least 8.

The heat carrier medium preferably comprises 50 to 90% by weight ofwater, more preferably 60 to 80% by weight of water.

The heat carrier medium may also comprise further customary ingredients,for example corrosion inhibitors, viscosity modifiers, biocides, etc.

The invention is illustrated in detail by the examples which follow.

EXAMPLES

The magnetocaloric material used was pulverulent MnFeP_(0.5)As_(0.5).

Example 1

10 g of the pulverulent material were stirred in 100 ml of dist. waterfor two weeks. An analysis shows that approx. 2% of the arsenic presentin the compound was dissolved.

Example 2

10 g of the pulverulent material was stirred in 100 ml of water withpH=11, which has been established with NaOH, for two weeks. An analysisshows that less than 0.12% of the arsenic present in the compound wasdissolved.

Example 3

10 g of the pulverulent material were stirred in a mixture of ethanoland water (1:1) for two weeks. An analysis shows that approx. 0.3% ofthe arsenic present in the compound was dissolved.

1.-14. (canceled)
 15. An aqueous heat carrier medium for magneticcoolers, magnetic heat pumps and magnetic generators, comprising 10 to70% by weight of water miscible alcohols, diols, polyols or mixturesthereof and which has a pH of at least
 8. 16. The medium according toclaim 15, comprising an alcohol selected from the group consisting ofmethanol, ethanol, n-propanol and 2-propanol.
 17. The medium accordingto claim 15, comprising a diol selected from group consisting ofethylene glycol, propylene glycol and butanediol.
 18. The mediumaccording to claim 15, further comprising a magnetocaloric material,which is part of a magnetic cooler, of a magnetic heat pump or of amagnetic generator.
 19. The medium according to claim 15, furthercomprising a magnetocaloric material selected from the group consistingof (1) compounds of the general formula (I)(A_(y)B _(y−1))₂₊₈C_(w)D_(x)E_(z)  (I) where A is Mn or Co, B is Fe, Cror Ni, C, D and E at least two of C, D and E are different, have anon-vanishing concentration and are selected from P, B, Se, Ge, Ga, Si,Sn, N, As and Sb, where at least one of C, D and E is Ge, As or Si, ι isa number in the range from −0.1 to 0.1, and w, x, y, and z are numbersin the range from 0 to 1, where w+x+z=1; (2) La— and Fe-based compoundsof the general formulae (II) and/or (III) and/or (IV)La(Fe_(x)Al_(1−x))₁₃H_(y) or La(Fe_(x)Si_(1−x))₁₃H_(y)  (II) where x isa number from 0.7 to 0.95, y is a number from 0 to 3;La(Fe_(x)Al_(y)Co_(z))₁₃ or La(Fe_(x)Si_(y)Co_(z))₁₃  (III) where x is anumber from 0.7 to 0.95, y is a number from 0.05 to 1−x, z is a numberfrom 0.005 to 0.5;LaMn_(x)Fe_(2−x)Ge  (IV) where x is a number from 1.7 to 1.95; and (3)Heusler alloys of the MnTP type where T is a transition metal and P is ap-doping metal having an electron count per atom e/a in the range from 7to 8.5.
 20. The medium according to claim 18, wherein the magnetocaloricmaterial is selected from at least quaternary compounds of the generalformula (I) which, as well as Mn, Fe, P and optionally Sb, additionallycomprise Ge or Si or As or Ge and As or Si and As, or Ge, Si and As. 21.A method comprising contacting a heat carrier medium comprising anaqueous liquid which has a pH of at least 12 at 25° C. with amagnetocaloric material.
 22. A magnetic cooler, magnetic heat pump ormagnetic generator, comprising at least one magnetocaloric material andan aqueous heat carrier medium, wherein the heat carrier mediumcomprises 10 to 70% by weight of water-miscible alcohols, diols, polyolsor mixtures thereof and has a pH of at least 8.